The development of assays for measuring the presence and amount of desired substances is highly desirable for a variety of purposes, including for medical, veterinary, research, and environmental uses. It is further desirable to design and isolate molecules having an activity which is regulatable by a desired substance. These regulatable molecules are useful to detect the amount and presence of a desired analyte, utilizing the ability of the analyte to directly or indirectly (e.g., by competition) regulate the molecule's activity.